Variable fluid delivery systems, i.e. systems for transporting a fluidized composition at a controllable flow rate, are useful in a wide variety of applications. One example of a system in which a variable fluid delivery system is utilized are sprayer systems used in agricultural implements which apply fertilizers, pesticides, herbicides, and other agricultural chemicals to fields ("agents").
One type of sprayer system generally regulates the amount of agent applied by diluting the agent in a carrier fluid and then controlling system pressure to apply the agent at a fixed predetermined rate. Specifically, the pressure is varied in accordance with the speed of the application vehicle. The volume of the sprayed combined agent/carrier fluid is regulated to vary with the speed of the application vehicle, and the amount of agent applied per unit of area is maintained at a relatively constant predetermined value.
Another type of sprayer system is an injection based system which controllably injects an agent into a carrier fluid at either a fixed or variable rate. Further, another type of sprayer system delivers a pressurized undiluted gaseous agent such as anhydrous ammonia.
Two types of feedback control systems have been utilized in the art in order to provide feedback for controlling the application rate in agricultural sprayer systems. The first type of feedback control mechanism is responsive to the measured pressure within the sprayer system. This type of system is generally disclosed, for example, in U.S. Pat. No. 3,877,645 issued to Oligschlaeger, U.S. Pat. No. 4,052,003 issued to Steffen, U.S. Pat. No. 4,392,611 issued to Bachman et al., and U.S. Pat. No. 4,803,626 issued to Bachman et al. The pressure responsive systems rely on the principle that the flow rate through a sprayer system may be calculated from the pressure in that system given known factors such as the number and size of the nozzles, the concentration of the agent in the carrier fluid, the specific gravity, etc. This type of system provides a very fast response time to pressure changes (because of the quick response of pressure sensors) which enables the control system to respond quickly to changes in the system. However, a pressure responsive system tends to have relatively poor accuracy since a number of the factors may change over time. For example, nozzle size may vary over time due to wear in the nozzles. In addition, pressure variations may exist between different points in the system plumbing, which also tends to corrupt the accuracy of pressure-based systems. Further, the accuracy of pressure responsive systems is relatively poor because pressure is an instantaneous measurement, requiring the flow rate itself to be calculated, rather than actually measured, which may introduce a degree of computational error to the resultant value obtained. Therefore, it is difficult to achieve accurate application rate calculations using pressure responsive systems, and such systems may require periodic recalibration to reduce the transient variations which may occur in the system over time.
The second type of feedback control mechanism relies on a flowmeter to provide a feedback signal for controlling the application rate of the sprayer system. These systems are used, for instance, in sprayer systems for applying anhydrous ammonia. Many commercially-available flowmeters are capable of providing accurate readings with regard to the application rate and total volume applied. However, flowmeters tend to have comparatively long response times, particularly at low flow rates, since most operate by providing a pulsed signal output with a frequency that varies proportionally with the rate of flow. Due to the comparatively long response time, the flowmeter responsive systems are not capable of quickly responding to changes in a system, and are therefore generally unsuitable for many control systems that must accommodate for variations in real time.
It is also known in the art to utilize flowmeters in sprayer control systems to serve a different purpose, as is shown for example in U.S. Pat. No. 4,803,626 issued to Bachman et al. In this type of application, a flowmeter is used as a "nozzle monitor" to detect failure conditions in the system such as plugged nozzles, worn-out nozzles or leaks in the plumbing.
In view of the above, it will be appreciated that the fast response of a pressure responsive control system is compromised by less than desirable accuracy, while the high accuracy of a flowmeter system is compromised by less than desirable response time. Therefore, a need exists in the art for a controller device for variable fluid delivery systems which provides high accuracy in combination with fast response time, without the drawbacks which have typically been associated with previous and conventional systems.